Ha. We could tighten this up further to require a non-blank immediately after the @ for a type application. Thus f @Int or g @(Tree Int) but not f @ Int g @ (Tree Int) That would leave (p1@p2) and (p1 @ p2) free for patterns. Simon From: ghc-steering-committee <ghc-steering-committee-bounces@haskell.org> On Behalf Of Richard Eisenberg Sent: 17 August 2018 17:18 To: Joachim Breitner <mail@joachim-breitner.de> Cc: ghc-steering-committee@haskell.org Subject: Re: [ghc-steering-committee] Discussion about "Type Application in Patterns" (#126) Note that this rule is documented, in brief, in the manual:
A visible type application is preceded with an @ sign. (To disambiguate the syntax, the @ must be preceded with a non-identifier letter, usually a space. For example, read@Int 5 would not parse.)
On Aug 17, 2018, at 12:16 PM, Richard Eisenberg <rae@cs.brynmawr.edu<mailto:rae@cs.brynmawr.edu>> wrote: The rule is this, implemented in the lexer: With -XTypeApplications on, the character immediately before an @ determines how it is lexed. 1. If the character is a legal end-of-identifier character (respecting the state of the -XMagicHash flag), then the @ is an as-pattern. 2. Otherwise, then the @ is the beginning of a type application. I abbreviate the rule as talking about whitespace sensitivity, because if the character preceding the @ is, say, a +, then we lex +@ as a user-defined operator. This rule has been implemented since GHC 8.0 and applies in patterns as well as expressions. I don't think anyone noticed. :) So it works reasonably well. It's a silly rule and I'd be happy to do better, but I don't think it's strictly necessary to aim for better here. It would be reasonable to warn when a user writes an as-pattern that doesn't conform to this rule (with -XNoTypeApplications). Richard On Aug 17, 2018, at 1:37 AM, Joachim Breitner <mail@joachim-breitner.de<mailto:mail@joachim-breitner.de>> wrote: Hi Richard, Am Donnerstag, den 16.08.2018, 22:28 -0400 schrieb Richard Eisenberg: Considering this future of relaxed ordering requirements on data constructors isn't compulsory, but I do think it's better if we don't paint ourselves into a corner around this. I agree. But so what do we do? Already now require that there is no space between @ and the following token when the user wants to use type applications in the pattern? And would we also require the user to add a space after an at-pattern? In which case we should probably start a deprecation cycle for the currently legal foo (a @b) = … Cheers, Joachim -- Joachim Breitner mail@joachim-breitner.de<mailto:mail@joachim-breitner.de> http://www.joachim-breitner.de/<https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.joachim-breitner.de%2F&data=02%7C01%7Csimonpj%40microsoft.com%7C549984ffda524c614d6008d6045cf7bd%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636701194744752293&sdata=i4KtV3B%2B2YUluqn3Rx7th3sm6%2BDYkinoMQE6n%2FNqdZQ%3D&reserved=0> _______________________________________________ ghc-steering-committee mailing list ghc-steering-committee@haskell.org<mailto:ghc-steering-committee@haskell.org> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee _______________________________________________ ghc-steering-committee mailing list ghc-steering-committee@haskell.org<mailto:ghc-steering-committee@haskell.org> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee